This invention relates to a transfer unit and an image forming apparatus using the transfer unit. More particularly, the invention relates to a transfer unit for forming an image on a recording carrier by a transfer material (toner), charging a transfer medium (paper) to a polarity opposite that of a charge on the transfer material by a corona discharge and transferring the transfer material to the transfer medium, as well as to an image forming apparatus using this transfer unit.
In a recording apparatus such as an electrophotographic printer, an optical image is projected upon a photosensitive drum to form an electrostatic latent image on the photosensitive drum, the electrostatic latent image is then developed into a toner image and the toner image is transferred to paper, whereby the image is printed.
FIG. 13 is a diagram showing the overall construction of an electrophotographic printer which uses laser light as the exposing light source. The printer includes a photosensitive drum 1 having a photoconductor (photoreceptor) on the surface thereof. The drum 1 is rotated in the direction of arrow A at a constant speed. The printer further includes a primary corona discharge device 2 for uniformly charging the surface of the photosensitive drum 1, an exposure optical unit 3 for irradiating the photosensitive drum 1 with an optical image to form an electrostatic latent image, a developing unit 4 for forming a toner image corresponding to the electrostatic latent image and having a toner supply section 4a and a developing section 4b, a transfer corona discharge device 5 for transferring the toner image to printing paper CP, an optical charge removing device 6 for irradiating the photosensitive drum with light to remove electric charge from the drum, a cleaner 7 having a brush 7a and a blade 7b for removing and wiping off toner remaining on the photosensitive drum, rollers 8, 9 for conveying the paper, and a fixing unit 10, constituted by a thermal fixing roller 10a or the like, for fixing the toner that has been transferred to the paper.
A detector 11 detects the printing paper CP. A high-voltage power supply 12 is equipped with a power supply section 12a for generating a corona discharge by applying a voltage V.sub.C to the primary corona discharge device 2, a power supply section 12b for charging the toner to a prescribed polarity by applying a developing bias voltage V.sub.B to a magnet roll (developing roll) of the developing section 4b, and a power supply section (constant-current source) for generating a corona discharge by applying a transfer voltage VT to the transfer corona discharge device 5. The paper CP, which is delivered one sheet at a time from a hopper (not shown) on the right side of the drawing, is conveyed in the direction of arrow B and is discharged into a stacker (not shown) on the left side of the drawing via the transfer discharge device 5 and fixing unit 10.
When the optical image is projected upon the surface of the photosensitive drum 1 uniformly charged to a positive charge, for example, by the primary corona discharge device 2, the potential at portions upon which the light is incident drops so that an electrostatic latent image is formed. Next, the magnet roll MGR biased at the prescribed developing voltage VB is rotated in the developing unit 4 so as to rub the positively charged toner against the surface of the photosensitive drum, whereupon the toner is dispersed over the electrostatic latent image to form a toner image. If the transfer corona discharge device 5 subsequently generates a corona discharge from the bottom side of the paper CP at a potential whose polarity (minus) is opposite the potential to which the toner image has been charged, the paper is charged negatively. As a result, the toner image is attracted to the paper CP and transferred thereto. The paper CP to which the toner image has been transferred by the transfer corona discharge device 5 is conveyed to the fixing unit 10. Here the paper CP is subjected to thermal fixing before being discharged into the stacker (not shown) on the left side of the drawing. After the toner image has been transferred to the paper, the photosensitive drum 1 is rotated further, charge is removed by the optical charge removing device 6 and residual toner is removed by the cleaner 7 to prepare for formation of the next electrostatic latent image. It should be noted that timing for starting and ending projection of the optical image by the optical unit 3 and timing for starting and stopping the corona discharge performed by the transfer corona discharge device 5 is controlled by a controller (not shown) using as a reference the time at which the leading edge of the paper is detected by the detector 1. This assures that the paper will be printed on correctly.
FIG. 14 is a diagram showing the construction of the exposure optical unit 3. The unit includes a laser diode 3a, a collimating lens 3b, a polygon mirror 3c which causes a laser beam to scan the photosensitive drum 1 in the longitudinal direction (along the direction of arrow C), an F-.theta. lens (image forming lens) 3d and a spindle motor 3e for rotating the polygon mirror at a constant speed.
The laser light is on/off modulated by controlling the on/off action of a laser diode 3a based upon dot-image printing information. The laser light on/off-modulated by the printing information arrives at the polygon mirror 3c via the collimating lens 3b. Since the polygon mirror 3c is being rotated by the spindle motor 3e at a constant speed, the incident laser light is moved repeatedly in the longitudinal direction (along the direction of arrow C) of the photosensitive drum 1 via the F-.theta. lens 3d. Accordingly, if the laser light on/off-modulated by the printing information is made to scan longitudinally of the photosensitive drum 1 while the drum is rotated in the direction of arrow A, a dot optical image will be projected upon the surface of the photosensitive drum to form an electrostatic latent image in the form of dots on the drum surface.
With reference again to FIG. 13, the transfer corona discharge device 5 has a corona discharge wire 5a to which a high DC voltage on the order of -4.about.-7 KV is applied so that a constant current will flow through it. As a consequence of this arrangement, there are occasions where leakage occurs between the discharge wire 5a and a chassis 5b of the corona discharge device 5 as well as between the discharge wire 5a and the photosensitive drum 1. Further, there are instances where electrically conductive foreign matter attaches itself to the discharge wire 5a, or where the wire 5a is contaminated or damaged, thus giving rise to leakage. When such leakage occurs, the impedance Z of the transfer corona discharge device 5 decreases.
When the transfer corona discharge device 5 is operating normally, the constant-current source 12c applies the high DC voltage V.sub.T (=V.sub.TN =i.sub.T .multidot.Z.sub.N) and control is performed in such a manner that a constant current i.sub.T will flow, as shown in FIG. 15A. When leakage occurs and the impedance drops from Z.sub.N to Z.sub.L, therefore, the applied voltage falls to VTL (=i.sub.T .multidot.Z.sub.L), as illustrated in FIG. 15B. When the applied voltage declines, the toner image that has been formed on the recording carrier, namely the photosensitive drum 1, is no longer transferred to the transfer medium or paper CP normally. Furthermore, in the event of a short circuit, the impedance Z declines further and the relation VTS=(i.sub.T .multidot.Z.sub.S) is established. This makes transfer impossible.
The prior art is devoid of effective means for detecting that transfer has been performed normally, i.e., for detecting that transfer has not been carried out normally (referred to as "transfer omission"). All that is provided is circuitry for protecting the constant-current source. Specifically, an overcurrent detecting circuit or overvoltage detecting circuit is provided. However, such circuitry includes a drive circuit for driving the constant-current source or a detecting circuit provided in order to protect the load (the transfer corona discharge device) from damage, and the relevant set values are made much higher than values which assure transfer. This means that transfer omission due to leakage cannot be detected.
Japanese Patent Application Laid-Open (KOKAI) No. 63-10167 discloses art in which an abnormality in the constant-current source used for the corona discharge of the transfer corona discharge device is detected by a fluctuation in the current value. That is, the power supply is judged to be abnormal when the transfer current which flows into the transfer corona discharge device fluctuates above a set value. Though power-supply abnormality can be detected with this known method, it is not possible to detect a discharge abnormality ascribable to leakage, i.e., transfer omission due to leakage. The reason is that even if the current which flows into the transfer corona discharge device varies slightly when discharge abnormality occurs owing to leakage, feedback is applied immediately and control is effected so as to render the current value constant. Holding the current value constant is the essence of the constant-current source.
Since transfer omission due to leakage thus cannot be detected immediately in the prior art, the occurrence of transfer omission is discovered by the user only after printing a number of sheets. This means that a large number of unsatisfactory copies may be produced. In other words, a problem encountered in the prior art is that even if transfer omission occurs, faulty printing is construed as being normal and unsatisfactory printout is performed.
Another problem is that detection of transfer omission due to leakage, namely detection of discharge abnormality, is delayed. This can lead to other abnormalities in the system.
Still another problem with the prior art is that even if unsatisfactory printing (transfer omission) is discovered, maintenance is troublesome because it is difficult to determine where the abnormality is occurring.